Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation, and increasing the mechanical compression ratio and delaying the closing timing of the intake valve as the engine load becomes lower at the time of engine medium and high load operation in the state holding the actual combustion ratio constant (for example, see Japanese Patent Publication (A) No. 2004-218522).
In such a internal combustion engine, if the mechanical compression ratio rises, the gap between the piston top and the combustion chamber inner wall surface at the end of the compression stroke will be narrowed, which causes a powerful squish flow in the combustion chamber. This as a result causes powerful disturbances in the combustion chamber, whereby good ignition and combustion of fuel can be acquired. However, if the mechanical compression ratio falls, the gap between the piston top and the combustion chamber inner wall surface at the end of the compression stroke will widen, so the squish flow will weaken or cease to happen. As a result, the disturbances occurring in the combustion chamber will be weakened and thereby ignition and combustion of fuel will deteriorate.